Once your destination is set and the map is up, touch the bottom left corner where it shows the travel time, then touch the icon that pops up on the bottom right that looks like direction arrows to your final place. That should bring up a screen that has 2 or 3 route directions for you to pick, along with distance and time info. Hopefully, the one you want will be there.
If that doesn't get your desired route, another way is to just set the destination to one of the cities/towns that the route you want to take would bring you through. Once you get there, reset the destination to your final spot. If it still re-routes you off your desired track, you can always keep giving it city destinations along the route you wan to travel.
Sometimes, you just have to force technology to work the way you want it to.